net: ipmr: Add MFC offload indication

Allow drivers, registered to the fib notification chain indicate whether a
multicast MFC route is offloaded or not, similarly to unicast routes. The
indication of whether a route is offloaded is done using the mfc_flags
field on an mfc_cache struct, and the information is sent to the userspace
via the RTNetlink interface only.

Currently, MFC routes are either offloaded or not, thus there is no need to
add per-VIF offload indication.

Signed-off-by: Yotam Gigi <yotamg@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>